This invention relates to electrical testing devices and is particularly concerned with a test instrument for checking the operability of arc fault circuit interrupters installed in electrical distribution circuits. One form of the invention can also perform tests for ground fault interrupters, load carrying capacity and proper wiring.
The common type of circuit breaker used for residential, commercial, and light industrial applications has an electro-mechanical thermal-magnetic trip device to protect against short circuit and overcurrent conditions. Some circuit breakers also include ground fault interrupters which trip in response to a line to ground fault. More recently, interest has arisen in protecting against arc faults as well. Arc faults are intermittent, high impedance faults, which can be caused by corroded, worn or aged wiring or insulation, loose connections, wiring damaged by nails or driven through the insulation and the like. Arc faults can occur in the permanent wiring, receptacles or in the wiring of loads or extension cords plugged into a receptacle. Arcing faults occur when electric current “arcs” or flows through ionized gas between two ends of a broken conductor, between two conductors supplying a load or between a conductor and ground. Because of their intermittent and high impedance nature, arc faults do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger standard circuit breakers. Nevertheless, arc faults present a fire hazard because they can generate sufficient heat to ignite insulation or adjacent combustible materials.
An arc fault circuit interrupter (AFCI) can sense the current passing through the line conductor of a branch circuit, process the sensed information to determine whether the characteristics of the line current represent the occurrence of an arc fault, and trip open the branch circuit in which an arcing fault has occurred. An example of such a device is shown in U.S. Pat. No. 5,682,101.
Upon installation of an AFCI, and periodically thereafter, the AFCI should be tested to verify that it is operating properly and will provide the intended protection. Some AFCI's provide a built-in test circuit. This approach has been criticized as testing only the trip mechanism of the AFCI and not its sensing circuitry. To overcome this situation plug-in testers have been developed which create a simulated arc fault condition remote from the AFCI. Examples of such devices are shown in U.S. Pat. Nos. 6,072,317, 6,218,844 and 6,313,642. While these devices provide the basic function of testing an AFCI, they have certain drawbacks which prevent them from fulfilling the need for a simple, effective, reliable and safe method and apparatus for testing an AFCI.